{"id":1657,"date":"2026-11-09T22:37:03","date_gmt":"2026-11-09T22:37:03","guid":{"rendered":"https:\/\/virtualserversvps.com\/blog\/?p=1657"},"modified":"2026-11-09T22:37:03","modified_gmt":"2026-11-09T22:37:03","slug":"kernel-hz-tickless-idle-vps-overhead","status":"publish","type":"post","link":"https:\/\/virtualserversvps.com\/blog\/kernel-hz-tickless-idle-vps-overhead\/","title":{"rendered":"Kernel HZ, NO_HZ and Tickless Idle: Why Idle vCPU Overhead Differs Between VPS Hosts"},"content":{"rendered":"
\n

Two VPS hosts can sell identical vCPU counts and differ by several percent on a latency-sensitive workload before you write a line of application code, because of how often the guest kernel wakes a sleeping core. The timer tick, the CONFIG_HZ value baked into the kernel, and whether the build uses NO_HZ_IDLE or NO_HZ_FULL decide how much work an idle vCPU does and how precisely it can sleep. On a hypervisor, those micro-wakeups also translate into VM exits that steal real cycles from the host.<\/p>\n

What the tick actually is<\/h2>\n

A classic kernel programs a local APIC timer to fire CONFIG_HZ times per second. Each interrupt runs scheduler accounting, RCU callbacks, and timekeeping, then returns to idle. At CONFIG_HZ=250 that is 250 interrupts per second per core even with nothing to do; at 1000 it is a thousand. On a 16-vCPU guest that is 16,000 wakeups per second of pure overhead.<\/p>\n

Idle cores in a VM are worse than idle cores on bare metal because each tick can trigger a VM exit and a return, which costs on the order of microseconds. Read the counter directly: grep -E 'LOC|RES|CAL' \/proc\/interrupts<\/code> before and after a 30 second idle window gives a wakeups-per-second figure you can compare between hosts.<\/p>\n

NO_HZ_IDLE versus NO_HZ_FULL<\/h2>\n

NO_HZ_IDLE stops the periodic tick when a CPU has nothing to run, keeping time through the clocksource on wake. Every general-purpose distribution kernel uses it; it removes most idle overhead automatically. NO_HZ_FULL goes further and removes the tick even while a single task runs, at the cost of per-CPU nohz_full=<\/code> configuration and reduced accounting precision.<\/p>\n

NO_HZ_FULL is for dedicated real-time or DPDK-style pinning, not for shared web hosting. What matters for a VPS is whether the provider’s kernel exposes a sane clocksource and whether \/sys\/devices\/system\/clocksource\/clocksource0\/available_clocksource<\/code> includes a stable option such as kvm-clock<\/code> or tsc<\/code>. A guest falling back to hpet<\/code> or acpi_pm<\/code> pays far more per timer read than the tick overhead you are trying to remove.<\/p>\n

Measuring idle overhead per host<\/h2>\n

Boot a clean instance with no services beyond sshd, wait two minutes for boot-time work to settle, then sample for 60 seconds: sar -u 1 60<\/code>, vmstat 1 60<\/code> and the interrupt counters. Record %system<\/code>, %idle<\/code>, interrupts per second and context switches per second. A guest with proper tickless behaviour shows nearly zero system time and only a handful of interrupts per second.<\/p>\n

Two samples are not enough \u2014 collect the same window three times and compare medians. Run the check on a second instance of the same plan to see host-to-host variance, and repeat after 24 hours of uptime, because some providers inject guest agents and monitoring probes that wake cores far more often than a tick ever would.<\/p>\n

Thresholds worth acting on<\/h2>\n

On a quiet 1-vCPU instance, expect under 200 interrupts per second, under 100 context switches per second, and %system<\/code> under 0.5. Sustained figures several times higher suggest a guest agent polling in a loop, a monitoring daemon with a tight interval, or a host that has disabled tickless idle in its kernel configuration.<\/p>\n

For a latency workload, run a small task pinned to an idle core with taskset -c 3 cyclictest -p 50 -i 1000 -n -l 10000<\/code>. Wakeup latency p99 above 100 \u00b5s on an otherwise idle VPS points to tick or VM-exit interference; on bare metal the same test typically stays under 30 \u00b5s. That gap is the tax you are paying for virtualisation.<\/p>\n

Building the kernel argument for comparison<\/h2>\n

You cannot change the host’s CONFIG_HZ, but you can compare guests fairly by noting zcat \/proc\/config.gz | grep -E 'CONFIG_HZ|CONFIG_NO_HZ|CONFIG_HIGH_RES_TIMERS'<\/code> where the provider ships \/proc\/config.gz<\/code>. Many do not, in which case use uname -a<\/code> plus the distribution default and state your assumption clearly in the report.<\/p>\n

Always verify high-resolution timers are active: cat \/sys\/devices\/system\/clocksource\/clocksource0\/current_clocksource<\/code> and grep hres_active \/proc\/timer_list<\/code>. Without hrtimers, tickless idle is effectively off and your measurements describe the distributor’s build, not the provider’s hardware. Correct for that before drawing conclusions about hosts.<\/p>\n

Isolating tick cost from neighbour noise<\/h2>\n

Idle overhead and CPU steal look similar in coarse tools. Separate them with mpstat -P ALL 1 30<\/code>: uniform system time across cores with no steal is tick overhead, while spikes concentrated on some cores with a rising %steal<\/code> is the hypervisor. Interrupt counters help too \u2014 tick interrupts arrive at a fixed cadence, host interference does not.<\/p>\n

For a decisive test, compare perf stat -e cycles,instructions,context-switches -a sleep 30<\/code> on an idle instance against a busy one. Cycles per second on the idle guest give you a direct overhead figure in hardware terms, which is the number that matters when you are paying for a vCPU that is supposed to be sleeping.<\/p>\n

What to do with the finding<\/h2>\n

If a provider’s idle overhead is high, you have three options: accept it, pin latency-critical work to a core and reduce the impact of wakeups with isolcpus<\/code>, or move. For most workloads the practical fix is to remove guest-side pollers, set monitoring intervals above 30 seconds, and use event-driven tooling instead of per-second sampling scripts that themselves generate thousands of wakeups.<\/p>\n

Record the idle overhead next to the price in your provider spreadsheet, because a host that costs ten percent less but burns a percent of every core on tick overhead and another on steal is not cheaper. Our VPS comparison table<\/a> notes kernel and clock-source details per tier, and the plan breakdown<\/a> explains which configurations are worth benchmarking for low-latency work. Rerun the same measurements after any provider migration.<\/p>\n<\/div>\n","protected":false},"excerpt":{"rendered":"

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